Navigation processing for a satellite positioning system receiver

ABSTRACT

A method for navigation processing in a satellite positioning system receiver is disclosed. A method in accordance with the present invention comprises separating the three SATPS satellites into a first pair and a second pair, constructing a primary solution and an alternate solution, wherein the primary solution and the alternate solution satisfy the measurement constraints, computing a Doppler difference estimate for the primary solution and a Doppler difference estimate for the alternate solution, computing Doppler difference residuals for the first pair and the second pair of SATPS satellites, and comparing the Doppler difference residuals for said primary and alternate solutions to determine a valid solution. Typically, the computing of a Doppler difference residuals comprises differencing a measured Doppler difference from an estimated Doppler difference for the first pair and the second pair of SATPS satellites. Typical determination of a valid solution comprises comparing the difference between the Doppler difference residuals to a predetermined number. Usually when the Doppler difference residuals exceed the predetermined number, the alternate solution is selected as the valid solution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U. S. C. §119(e) of U.S.Provisional Patent Application No. 60/228,965, filed Aug. 29, 2000,entitled “NAVIGATION PROCESSING FOR A SATELLITE POSITIONING SYSTEMRECEIVER,” by Keith J. Brodie et al., which application is incorporatedby reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to receiver operation insatellite positioning systems (SATPS) such as the Global PositioningSystem (GPS), GLONASS, and the proposed Galileo system, and inparticular to navigation processing for a SATPS receiver.

2. Description of the Related Art

Navigation processing for SATPS receivers usually comprises processingpseudoranges and Doppler frequency shift measurements to determineposition and velocity of the receiver. However, this approach issometimes unable to distinguish between alternate (valid and invalid)three-satellite solutions. Examples of such processing techniques arefound in U.S. Pat. No. 5,416,712 by Geier, U.S. Pat. No. 5,883,595 byColley, and U.S. Pat. No. 5,590,043 by McBurney, all of which areincorporated by reference herein.

SUMMARY OF THE INVENTION

To minimize the limitations in the prior art, and to minimize otherlimitations that will become apparent upon reading and understanding thepresent specification, the present invention discloses a method andapparatus for navigation processing in a satellite positioning systemreceiver, including methods and apparatus for generating a least-squaresfix, checking the validity of measurements, and processing measurementsin a low-power mode which involves repeated on/off cycles of thereference oscillator used to make the measurements.

A method in accordance with the present invention comprises separatingthe three SATPS satellites into a first pair and a second pair,constructing a primary solution and an alternate solution, wherein theprimary solution and the alternate solution satisfy the measurementconstraints, computing a Doppler difference estimate for the primarysolution and a Doppler difference estimate for the alternate solution,computing Doppler difference residuals for the first pair and the secondpair of SATPS satellites, and comparing the Doppler difference residualsfor said primary and alternate solutions to determine a valid solution.Typically, the computing of a Doppler difference residuals comprisesdifferencing a measured Doppler difference from an estimated Dopplerdifference for the first pair and the second pair of SATPS satellites.Typical determination of a valid solution comprises comparing thedifference between the Doppler difference residuals to a predeterminednumber. Usually when the Doppler difference residuals exceed thepredetermined number, the alternate solution is selected as the validsolution.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 illustrates the geometry of alternate solutions for a threesatellite fix;

FIG. 2 illustrates the geometry of the projection vector used to computethe initial point for the least-squares iteration for the alternate fixsolution;

FIG. 3 is the flow chart for the least-squares fix decisions on postingcomputed fixes and testing for alternate solutions;

FIGS. 4A and 4B are the flow chart for initial measurement validitychecking with an adaptive form of a signal power level test;

FIG. 5 is the flow chart for the decision to use, or not use DGPScorrections to GPS measurements;

FIGS. 6A, B, and C are the degraded mode logic for SATPS receiver;

FIG. 7 is the Receiver Manager to Navigation Library Interface;

FIGS. 8A through 8G is the flowchart for the main one-hertz navigationprocessing loop;

FIG. 9 Shows the navigation mode decision with an overview of theprocessing in each mode;

FIG. 10 is a flowchart of the mode decision logic for the mainnavigation loop.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Least-Squares Navigation Fix

A least-squares iterative solution for position and clock bias with fouror more satellites is well known. A description of the basicleast-squares algorithm for GPS pseudoranges can be found inUnderstanding GPS: Principles and Applications by E. Kaplan (ed.),Artech House, 1996, pp. 43-47 which is herein incorporated by reference.A three-satellite solution can also be computed by using altitude as afourth measurement. If the altitude is known it may be used, if not, adefault altitude may be assumed for receivers designed for terrestrialuse. Altitude may be known from user input or from a saved altitude froma prior period of navigation with more than three satellites. There areusually two solutions to the three-satellite problem. A diagram of thesituation is shown in FIG. 1. The sphere is a representation of aconstant altitude surface over, or under, the Earth's surface. In otherwords, every point on the sphere satisfies the altitude constraint. TheEarth's surface is not spherical of course, but close enough so that thediagram illustrates one instance of alternate three satellite solutions.The two points identified on the sphere satisfy the pseudorangeconstraints with different values for user clock bias, or we could say,equivalently, that the difference in the ranges to the satellites is thesame for the two alternative fixes.

In order to safely report and use a three satellite least-squaressolution, we must be able to determine if the solution we have computedis the correct one from the two possibilities. We have additional datato assist in checking the validity of the solution. The difference indoppler frequencies of the satellites in track observed, versus thatpredicted from the known satellite orbits and the computed solution is atest on the validity of a computed root. The doppler differenceresiduals are used to remove the error in the receiver's clock frequencyfrom the test, so the test has finer resolution and is generallyapplicable under a wider range of circumstances than a test on absolutedoppler.

The doppler difference residual is computed by differencing the observeddoppler frequencies between two satellites in track. This difference isindependent of the frequency error in the SATPS receiver's oscillator bywhich the received signal's doppler is measure. An estimate of this samedoppler difference is made by assuming the SATPS receiver is at thelocation of a proposed fix, and computing the doppler for the knownsatellite orbits by computing the dot-product of their earth-fixed framevelocity vectors with the line-of-sight vectors from the fix location tothe satellite positions. The doppler difference residual is then themeasured difference between doppler frequencies for two satellites minusthe estimated difference between dopler frequencies for two satellites.The sum of the absolute value of the doppler differences is then checkedagainst thresholds for reporting and testing alternate roots. The sum ofthe squared residuals could be used equally well with appropriatechanges in the threshold values used to make the reporting and testingdecisions.

Unfortunately, the differenced doppler residuals between two alternateroots are not always large enough to guarantee proper root selection.Any velocity of the receiver, in addition, is another unknown in thedoppler difference residuals which will limit the ability to decidebetween roots. In order to safely use one of the roots, the second rootshould be computed and checked. If intial user velocity is known, orsensor data such as a wheel speed sensor and a compass or headingestimate mantained by a gyro is available, the speed data can beincluded in the relative velocity computation used for generating theestimates.

A fast method to compute the second root has been found using aprojection of the first root through the plane of the satellites. Thismethod results in an approximate location for the second root, which isused to initialize the least-squares iterator so that it will close onthe alternate root. The position of the three satellites used in thesolution is used to form a plane. A line is drawn normal to this planeand intersecting the first root found on the surface of the constantaltitude sphere. This geometric construction is shown in FIG. 2. Thereare cases where the plane of the satellites does not intersect thesphere, nevertheless there may be two points on the normal to the planethat intersect the sphere. The projection point is computed from theinitial root, the position of the satellites, and the radius of theconstant altitude sphere using the following method.

S₁, S₂, S₃: Satellite position vectors

R₁: First root computed from least-squares iterator

Satellite position vector differences D₁=S₁−S₂

D ₂ =S ₂ −S ₃

Compute a projection vector, normal to the plane defined by the threesatellites.

P=D ₁ ×D ₂

Compute the unit-normal projection vector. $\hat{P} = \frac{P}{P}$

The form of the second, projected root is assumed to be

 R ₂ =R ₁ +α{circumflex over (P)}

where α is an unknown constant to be solved for. Since the second rootis also required to lie on the constant altitude sphere:

|R ₂ |=|R ₁ |=|R ₁ +α·{circumflex over (P)}|

This results in a quadratic for α with one non-trivial root:

α=−2R ₁ ·{circumflex over (P)}

The projected root is then

R ₂ =R ₁−2(R ₁ ·{circumflex over (P)}){circumflex over (P)}

R₂ is then used as the initial position estimate for the least-squaresestimator, which will normally close on the alternate root. Occasionallythis method may fail to find an alternate root to check. In this case,the three satellite fix is rejected, rather than reporting a fix whichmay be substantially in error. In some cases, the doppler differenceresiduals observed at the first root and the alternate root may notdiffer by a substantial amount. In this case, again lacking clearinformation to choose, the fix is rejected rather than reporting a fixwhich may be substantially in error.

Measurement Validity Testing

There are many potential sources of error in SATPS system measurements.These include noise, tropospheric and ionospheric delays, multipath,errors in syncing to the CDMA code used to modulate the satellitetransmissions, auto-correlations with the wrong portion of the code,cross-correlations with a different code from another satellite,interfering signals from the local environment or other satellitesystems, frequency alias in the tracking loops, and others. Errors suchas receiver measurement noise, atmospheric correction errors, errors inthe predicted satellite positions are well understood and modeled in thenavigation processing. The other errors, such as code-sync errors,multipath, and auto- and cross-correlations are not well modeled, can bevery large, and would severely affect navigation processing ifmeasurements affected by these errors are not identified and removedfrom the set of measurements to be processed. Accordingly, a strong setof validity tests has been developed to insure the integrity of themeasurements prior to processing. Those tests can largely be dividedinto those that depend on reasonable knowledge of the current navigationstate, and those that are independent of the state estimate and that cantherefore be applied in all conditions, whether prior navigation dataexists or not.

One of the novel test concepts utilized was the creation of an adaptivepower mask. The basic power mask test is a check on the carrier to noisedensity, or C/N₀ that the signal was tracked at to insure a reasonableprospect that the measurement is valid. This basic test has beenmodified to make it adaptive in two ways, as shown in FIG. 4. First, thethreshold power level is adapted to the current navigation mode. If theSATPS receiver is navigating, there are a number of subsequentmeasurement validity checks that will be performed, so it is reasonableto lower the power threshold in the validity test to attempt to maintainthe largest possible number of available good measurements. If, however,the receiver is not navigating, the power threshold is kept higher toprevent measurements with errors from getting into the initial solution.The second adaptation mechanism is to vary the form of the test betweenan average and minimum power test. The average power test is lessrestrictive, will allow more measurements, and has increased reliance onsubsequent testing to find measurement errors. The minimum power test ismore restrictive, requiring that all of the 100 millisecond bins used togenerate the measurement have power levels above the threshold. Sincethe 100 millisecond estimate of C/N₀ is noisy, this test will result inthe occasional rejection of measurements from signals near the thresholdfrom C/N₀ estimate noise alone, but this is a reasonable trade-off tomake against the risk of a measurement with large errors being used inthe initial navigation fix.

Following the loop defined in FIG. 4 the set of valid pseudoranges abovethe power threshold has been identified. The set of validdelta-pseudoranges are identified as those coming from the samemeasurement channels as the valid pseudoranges with the additionalcriteria that the coherent integration time for the carrier signal benon-zero.

A test is performed on the absolute value of the differences in thedelta-pseudoranges. This test is independent of the current navigationmode and can be applied without any knowledge of the SATPS receiver'sposition, the time, or the oscillator frequency error. In this test, thedifference between to delta-pseudorange measurements is compared to theabsolute physically possible limit for a terrestrial user given theconstellation of satellites in use by the SATPS. In the current GPSorbits, for example, the maximum doppler to a stationary terrestrialuser is about 800 m/s, so that the maximum doppler difference, observedbetween two satellites tracked simultaneously, must be less thanapproximately 1600 m/s. User velocity, taken to be a small numberrelative to these satellite dopplers, does not change the limitsubstantially because a significant user velocity towards one satellitewould be substantially away from another satellite which was far enoughaway (in azimuth to the receiver) to be close to the limit. Thus allavailable differences of valid delta-pseudoranges are computed andcompared to the 1600 m/s threshold. For any pair that violates thethreshold, a counter for each of the participants in the offendingdifference is advanced. At the end of the check, a single badmeasurement may have caused large differences in multiple pairs, but thecounters will identify the culprit assuming sufficient satellites areavailable. If any measurement is involved in more than one baddifference, it is rejected, and if no bad measurement, for example, withtwo valid delta-pseudoranges with a bad difference, they are bothrejected. The fault has been identified but not isolated.

A completely analogous check is made on the pseudorange measurements.For GPS, the physical range from a satellite to a SATPS which canreceive the signal is limited to a range from about 20 million metersfor a satellite overhead to about 26 million meters on the horizon.Without reference to the navigation state, the time of day, or any otherstate estimate, an absolute check on pseudorange measurements can beperformed. No two GPS pseudoranges recorded at the same time for aterrestrial STAPS receiver can differ by more than 6 million meters.Counters for bad differences are kept as in the delta-pseudorangedifference check to identify faulty measurements from in a set ofdifference checks.

Low-Power Measurement Processing

In low-power processing the reference oscillator is turned-off betweenmeasurement updates to save power. As a result, substantial oscillatordynamics, due to heating, are present in each set of measurements. Thesereference oscillator dynamics adversely affect normal processing of thedelta-pseudorange measurement in a Kalman filter because of theirunmodeled character. The filter must assign these unmodeledaccelerations to either the clock drift term or velocity terms in it'sstate update, and errors are inevitable. In order to work around thisproblem, differenced delta-pseudorange measurements are processedinstead of the whole delta-pseudorange. This limits the observability ofclock drift in the filter and results in substantially better velocityestimates than would be otherwise obtainable.

Conclusion

A novel method for navigation processing in a satellite positioningsystem receiver has been disclosed.

A method in accordance with the present invention comprises separatingthe three SATPS satellites into a first pair and a second pair,constructing a primary solution and an alternate solution, wherein theprimary solution and the alternate solution satisfy the measurementconstraints, computing a Doppler difference estimate for the primarysolution and a Doppler difference estimate for the alternate solution,computing Doppler difference residuals for the first pair and the secondpair of SATPS satellites, and comparing the Doppler difference residualsfor said primary and alternate solutions to determine a valid solution.Typically, the computing of a Doppler difference residuals comprisesdifferencing a measured Doppler difference from an estimated Dopplerdifference for the first pair and the second pair of SATPS satellites.Typical determination of a valid solution comprises comparing thedifference between the Doppler difference residuals to a predeterminednumber. Usually when the Doppler difference residuals exceed thepredetermined number, the alternate solution is selected as the validsolution.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention not be limited by this detailed description, but by the claimsappended hereto.

What is claimed is:
 1. A method of verifying the validity of a SATPSthree satellite solution, comprising: separating the three SATPSsatellites into a first pair and a second pair; constructing a primarysolution and an alternate solution, wherein the primary solution and thealternate solution satisfy the measurement constraints; computing aDoppler difference estimate for the primary solution and a Dopplerdifference estimate for the alternate solution; computing Dopplerdifference residuals for the first pair and the second pair of SATPSsatellites; and comparing the Doppler difference residuals for saidprimary and alternate solutions to determine a valid solution.
 2. Themethod of claim 1, wherein the computing of a Doppler differenceresiduals comprises differencing a measured Doppler difference from anestimated Doppler difference for the first pair and the second pair ofSATPS satellites.
 3. The method of claim 2, wherein the determination ofa valid solution comprises comparing the difference between the Dopplerdifference residuals to a predetermined number.
 4. The method of claim3, wherein when the Doppler difference residuals exceed thepredetermined number, the alternate solution is selected as the validsolution.
 5. A method of verifying the validity of a SATPS threesatellite solution, comprising: separating the three SATPS satellitesinto a first pair and a second pair; constructing a primary solution andan alternate solution, wherein the primary solution and the alternatesolution satisfy the measurement constraints; computing a Dopplerdifference estimate for the first pair and the second pair of both theprimary and alternate solutions; computing Doppler difference residualsfor the first pair and the second pair of SATPS satellites in eachsolution by subtracted measured doppler differences from the dopplerdifference estimates; and comparing the Doppler difference residuals forsaid primary and alternate solutions to determine a valid solution.